One of the main challenges in hot beverage machines, such a single-serve coffee machines and espresso coffee machines, is to provide hot water in an energy-efficient manner at sufficiently high flow rates. For brewing coffee or espresso coffee a brewing temperature of around 90-95° C. is needed. Given the fact that tap water is usually used as inlet liquid, this means that such a liquid heating device in a coffee or espresso machine usually has to heat up water from an inlet temperature of around 15-20° C. up to an outlet temperature of around 90-95° C.
Two different types of liquid heating system for such hot beverage machines are generally known. A first machine type heats up the water by means of a water boiler. The second machine type uses one or more flow-through heaters or thermoblocks. Machines using water boilers are most suitable for high flow rates at low pressures, which is especially suitable for the production of a “regular” coffee drink typically having a volume of around 120 ml. Machines using flow-through heaters are on the other hand most suitable for lower flow rates at high pressures, which is particularly suitable for the production of an espresso coffee having a typical volume of around 40-60 ml.
It becomes especially challenging if one and the same liquid heating device shall be used for a combined machine that allows to produce “regular” coffee drinks as well as espresso coffees. For “regular” coffee drinks the water should be provided at high flow rates and at low pressures, while a comparatively low flow rate and high pressures are needed in order to produce a crema which is highly appreciated for espresso coffee drinks. Therefore, neither the state of the art boiler nor the state of the art flow-through heating techniques seem to be suitable for such a combined coffee and espresso machine.
A further difference of the two types of state of the art liquid heating techniques is their different level of power consumption. While boiler heating systems usually have power levels ranging around 1,450 W, modern thermoblocks would, due to their inherent flow rate limitation, have to be operated at around 2,100 W in order to produce the needed flow rates of around 5-6 ml/s, in particular of 5.6 ml/s. Even though thermoblocks are generally preferable due to their capability to cope with higher water pressures, the above-mentioned high power levels make the thermoblocks not suitable to be used in low voltage countries, such as Brazil and the USA.
On the other hand, simply using a high pressure boiler system is also not recommended for mass-produced products, because such high pressure boiler systems are quite complicated and expensive to manufacture. Apart from that, there are very high safety requirements that are put on such systems which can more easily be met by using flow through heaters instead of high pressure boiler systems.
EP 2 481 329 A1 discloses an attempt to produce high flow rates with a regular flow-through heater. In order to overcome the inherent flow rate limitation of such a flow-through heater, EP 2 481 329 A1 proposes to pre-heat the water reservoir by means of the flow-through heater in advance, such that the inlet temperature of the water within the reservoir is pre-heated to a temperature of around 40° C. As soon as a coffee drink has to be produced, the water “only” has to be heated starting at 40° C. up to the water dispensing temperature of around 90-95° C. This enables to reduce the time taken to heat up the water to the dispensing temperature on demand. However, constantly heating up the water reservoir to a temperature of 40° C. also seems to be quite energy inefficient, especially if hot water is only needed seldom, e.g. if a coffee machine is only used once or twice per hour. This system is especially inefficient if the user only brews small amounts of coffee, e.g. 1 or 2 cups, while the full capacity of the water container is pre heated
Thus, there is still room for improvement.